Deja vu

Each lived moment usually feels unique. But about two-thirds of us occasionally experience the fleeting sensation that life is on repeat. It's as if we've been in exactly the same place having exactly the same experience before, even though we know that's an impossibility.

Known as deja vu (literally "already seen"), an early explanation was that the feeling arises from a delayed signal arriving from one of the eyes after a scene has already been processed by the brain. However, that idea was seemingly refuted by the case of a blind man who experienced deja vu.

Modern theories focus on the memory process of familiarity, as distinct from knowing – like when you recognise a face but can't quite place it. Perhaps certain scenes contain features, such as a wallpaper pattern, that trigger a feeling of familiarity.

Other clues come from people with temporal lobe epilepsy who experience deja vu before or during a seizure, and from dementia patients and others with "chronic deja vu", who feel every waking moment has already happened. All these cases involve damage to, or abnormal functioning in brain regions associated with recall and familiarity.

Flashbulb memories

The grandfather of American psychology, William James, wrote: "An impression may be so exciting emotionally as almost to leave a scar upon the cerebral tissues." This is how experts used to think about "flashbulb memories", so‑called because of their remarkable vividness.

The first study of the phenomenon was published by FW Colgrove in 1899 after he noticed the amazing detail with which people could recall the time they heard of President Lincoln's assassination. Modern research has confirmed that memories for emotionally intense situations are unusually vivid and detailed, but it has also shown that they are no more accurate than mundane memories.

Jennifer Talarico and David Rubin at Duke University recruited dozens of US students on 12 September 2001 (the day after the terror attacks) and asked them to write down their memories from the previous day and from 10 September. Months later, the students recalled the events of those days. Crucially, their accounts of 9/11 showed just as much evidence of forgetting as their accounts of the previous day. What made memories for 9/11 special was their subjective quality – the students were more confident in their memories, and felt as if they were reliving what had happened.

Tip-of-the-tongue phenomenon

There's nothing more frustrating than being taunted by an elusive word as you chase it around the corridors of your mind. Each time you're about to get your hands on it, the lexical pest sneaks down a proverbial mind hole.

According to the psychologists Amy Warriner and Karin Humphreys, the worst strategy in such situations is to keep hunting. The reason, they explain, is that tip-of-the-tongue states arise when we look for a word in the wrong part of our memory banks, and the more we persist, the more entrenched that mistaken habit becomes, thus making us more likely to look there next time around. The pair tested this idea by deliberately inducing tip-of-the-tongue states in student volunteers by asking them to name obscure objects such as an abacus. If they left a student in this state for 30 seconds, rather than just 10, before putting them out of their misery, the student was more likely to experience a tip-of-the-tongue state for that word when tested again two days later.

The lesson from this research is that we should deal with elusive words by looking them up immediately, if we can, or asking someone. That way they're less likely to evade us next time.

Deliberate forgetting

Wouldn't it be useful if our minds had their own refuse collection service – a way of selectively depositing those memories we no longer require while keeping hold of those that we do?

When it is under conscious control, psychologists call a system such as this "deliberate forgetting", and there is indeed evidence that it exists. To test this in the lab, participants are usually given lists of words and told to remember some but to forget others. Later, their memory is tested for all the words and it's been found that the to-be-forgotten words are recalled less well than the to‑be-remembered words.

Research is ongoing to find out whether this deliberate forgetting occurs at the storage phase of memory, perhaps via suppressed rehearsal, or later, at the retrieval phase. Of course, forgetting unwanted memories could also occur subconsciously – a mechanism Freud called repression. Here the evidence is less supportive. Traumatic memories that you'd think would be ideal candidates for repression are often particularly vivid and can be experienced as disturbing flashbacks.

Also seeming to contradict Freud, lab experiments have shown that while neutral words can be deliberately forgotten, emotionally charged words such as "incest" seem to be immune from this process.

Nostalgic reveries

Indulging in sentimental thoughts about the past used to be considered a sign of psychological weakness, similar to homesickness. However, the psychologists Clay Routledge, Constantine Sedikides, Tim Wildschut and their collaborators have overturned that view in recent years, with a series of studies demonstrating the manifold benefits of nostalgic reverie.

For example, they provoked people into feeling lonely and observed how it increased their tendency to feel nostalgic. In turn, this nostalgia left the participants feeling more loved and in a better mood.

Nostalgia also protects us from existential angst. Wildschut's team reminded a group of people of death (by asking them to think about their own decaying corpse!) and then gave them the chance to be nostalgic. Compared with thinking about an ordinary event from autobiographical memory, nostalgic reverie led people to have fewer death-related thoughts.

Most recently, the team found that being nostalgic led to people categorising positive traits as relevant to themselves more quickly than when thinking about a positive future event, perhaps because such memories often feature the self in a positive light. Writing in 2008, Wildschut and co argued that nostalgia is "emerging as a fundamental human strength".

Memory episodes

How is the unfolding story of our lives organised by our memories into discrete chapters? A clue came from a study published earlier this year by Gabriel Radvansky, at the University of Notre Dame, and his co-workers. They asked people to navigate through a virtual reality network of rooms, picking up and depositing objects on tables as they went. Their memories were tested periodically – the name of an object appeared on screen and the participants had to remember if it was the object they were currently carrying (held objects couldn't be seen) or the one they'd just put down. The key finding was that memory performance was far poorer after passing through a doorway into a new room than after covering the same distance within a single room, even though the same amount of time had lapsed.

The findings were replicated in a maze of real rooms. Radvansky's team said this showed how the act of passing through a doorway creates a new chapter in our unfolding memories. Presumably this finding could help explain that annoying experience of a walking into a room only to forget what you went there for.

False memories

The mind doesn't maintain immutable archives. Each recalled memory is a fresh reconstruction of what happened. This is a creative process, highly prone to suggestion. If you were a witness in a criminal case asked to recall how close two suspects were standing next to each other "whispering", you'd likely report a smaller distance than if the questioner had used the word "talking".

By asking participants to reflect and elaborate on childhood experiences – some real, some fabricated – the doyenne of false memory research, psychologist Elizabeth Loftus, has led people to develop wholly false memories, including strawberry ice-cream once making them sick and that they'd once met Bugs Bunny at Disneyland, an impossibility since Bugs Bunny is a Warner Bros character.

We're all vulnerable to false memories. Hillary Clinton once described the time she arrived at Bosnia under sniper fire. In fact, video evidence shows her arrival was peaceful. A mass experiment conducted by Slate magazine used images of events that never happened, such as Obama shaking hands with Iranian president Ahmadinejad. Twenty-five per cent of 5,000 readers said they recalled the meeting. "The Chicago Trib had a big picture," said one.